1. Field of the Invention
The invention relates to a digital control system, comprising an actuator for influencing a state variable in a process, a sensor for detecting the state variable, a converter for converting the detected state variable into a series of digital measured signal values, a digital signal processor for deriving an actuator control signal from the series of measured signal values.
The invention further relates to a digital sub-circuit to be used in the digital control system.
2. Description of the Related Art
A digital control system as described is known, for example, from European Patent Application 0 390 467. The digital control system described in that document relates to focus control and tracking control in an optical recording and/or reproducing apparatus, in which an optical record carrier is scanned by a radiation beam for recording and/or reproducing purposes. The focus error signal and the tracking error signal are then derived from the radiation beam reflected by the record carrier. The magnitude of the focus error signal and the tracking error signal is strongly related to the intensity of the reflected radiation beam. This intensity strongly depends on the intensity of the radiation source used and on the reflection of the record carrier. Since these parameters may vary considerably there is a need for rendering the loop gain of the control system electronically adjustable and/or normalizing the error signals by dividing the focus error signal by the intensity of the detected radiation beam.
Optical recording and/or reproducing apparatus are know in which the normalization and/or adjustable gain are realised by means of analog multiplication using analog sub-circuits. In optical recording and/or reproducing apparatus having digital focus and tracking controls, there is a problem in that digital multiplier circuits and/or digital sub-circuits are necessary for gain adjustment and/or normalization in digital control loops. The prior art digital multiplier circuits and the digital sub-circuits, however, are disadvantageous in that they need very many components. This is especially a disadvantage in integrated circuits because the large number of components requires a relatively large chip surface and so results in increased cost of such an integrated circuit.